Butyl rubbers such as isobutene-isoprene rubber (also denoted as IIR) and their halogenated analogues are an important class of synthetic rubbers.
In the conventional process for producing brominated butyl rubber (also denoted as bromobutyl rubber or BIIR) e.g. isobutene and isoprene are first polymerized in a polar halohydrocarbon medium, such as methyl chloride with an aluminum based initiating system, typically either aluminum trichloride (AlCl3) or ethyl aluminum dichloride (EtAlCl2). The butyl rubber does not appreciably dissolve in this polar medium, but is present as suspended particles and so this process is normally referred to as a slurry process. Residual monomers and polymerization medium are then steam stripped from the butyl rubber, before it is dissolved in a bromination medium, typically a non-polar medium such as hexane. The bromination process ultimately produces the final brominated product. The conventional process therefore employs separate polymerization and bromination steps employing two different media. The use of a polar medium for polymerization and a non-polar medium for bromination necessitates intermediate stripping and dissolving steps and is inefficient from an energy point of view.
The step of separating the monomers and methyl chloride from the butyl rubber is conducted before bromination in order to avoid the formation of highly toxic byproducts from the reaction of bromine with residual monomers. The normal boiling points of the components used in the process are: methyl chloride, −24° C.; isobutylene, −7° C.; and isoprene, 34° C. Any stripping process that removes the heavier of the residual monomers (isoprene) will also remove essentially all of the methyl chloride and isobutylene. The process of removing all of the un-reacted components from the rubber slurry requires significant amounts of energy. The greater molecular weight (and therefore higher boiling point) of the brominated monomers also precludes the removal of these species following the bromination process.
Solution processes for the polymerization of butyl rubber have been known for many years and are described for example in CA 1,019,095 and U.S. Pat. No. 3,361,725, where iso-pentane and n-hexane are used as polymerization medium and WO 2010/006983, which discloses the use of an aliphatic medium comprising at least 50 wt.-% of one or more aliphatic hydrocarbons having a boiling point in the range of 45° C. to 80° C. at a pressure of 1013 hPa as the preferred polymerization medium.
WO 2010/006983 further discloses a subsequent standard halogenation process where bromine is used as bromination agent. A major inefficiency of this process is that the theoretical fraction of bromine present in the reaction mixture which can be introduced into the polymer is at maximum 50% of the theory, and the actual utilization observed in commercial plants is usually less than 45%. Most of the remaining bromine is lost due to formation of hydrogen bromide as a by-product which, under normal conditions, does not brominate the polymer any further. Hydrogen bromide is subsequently neutralized with a basic material such as sodium hydroxide solution and washed off the bromobutyl rubber, as described for example in U.S. Pat. No. 5,077,345. As a consequence, large amounts of diluted alkali metal bromides or alkaline earth metal bromides are disposed off every year.
A known method to enhance the bromine utilization during butyl rubber bromination involves the application of at least 0.5 mol per mol of brominating agent of an oxidizing agent such as hydrogen peroxide or alkali or alkaline earth metal hypochlorite, optionally in the presence of an emulsifier which reoxidizes the hydrogen bromide back to elemental bromine. The regenerated bromine is thus available for further bromination of butyl rubber, thereby significantly increasing the bromine utilization. Such processes are disclosed for example in U.S. Pat. No. 3,018,275, U.S. Pat. No. 5,681,901 and EP 803 517 A. The utilization of bromine in U.S. Pat. No. 5,681,901 is at maximum 84% for emulsions comprising an oxidizing agent and at maximum 73% for experiments without emulsifier. The solvent used for bromination was cyclohexane.
EP 709 401 A discloses a process for improving the bromination efficiency in rubber bromination processes by carrying out the bromination reaction in the presence of elemental bromine and an aqueous solution of an organic azo compound such as azodiisobutyronitrile and/or an alkali or alkaline earth metal hypochlorite. However, there still remains a need for an efficient, environmentally favourable process for the preparation of bromobutyl rubbers that significantly reduces energy and raw material consumption and simultaneously allows to obtain brominated rubbers with a desirable microstructure.